In automotive engineering, information regarding the presence, the distance, and possibly the speed of objects is particularly needed for various control systems. Examples of such control systems or driver-assistance devices include automatic ranging, a pre-crash sensory system that triggers the airbags in a timely manner, lane-changing devices, or park-distance control devices. In this context, various distance sensors based on different physical principles are, in turn, conventional, such as laser, radar, or ultrasound. Laser and/or radar sensors are almost exclusively used in the application field of automatic ranging sensors, a combination of sensors utilizing the specific advantages of the sensors being especially favorable. In the case of automatic ranging systems or lane-change assistance devices, a fixed, single-point scan of the front traffic space is not sufficient, but rather, a certain sector must at least be scanned in order to reliably detect an object. Such sector-shaped emission is inherent to the radar sensor because of the radiation characteristic of its antenna, whereas, in the case of a laser sensor, this must be done actively by moving the laser or an optical system. In so doing, the laser beam is successively swept across the desired sector and scans it for objects. Since safety distances as long as, for example, 50 m are sometimes necessary, the laser must have an appropriately large range. For this purpose, the laser must have a correspondingly high intensity, i.e., it must be operated at a high beam power. However, this results in considerable power losses in the laser sensor, which must be supplied by an energy source and must be dissipated in the form of heat, using appropriate cooling measures. If passive cooling measures, such as heat sinks, do not suffice in this case, then active cooling systems requiring additional energy must be used.
On the other hand, the power output of laser sensors is limited by safety requirements for the benefit of persons in the vicinity of the vehicle, who can be struck by the laser beams and receive an eye injury due to a reflex.
German Published Patent Application No. 39 03 501 describes an optical distance-measuring device for vehicles, which includes a semiconductor laser as an emitter for the very short infrared range. The emitting capacity of the semiconductor laser is automatically adapted to the environmental conditions, especially visibility, by a signal evaluation unit, and is adjusted to conform to eye-safety requirements. The adjustment of the power output of the system is based on the received signal. This means that the emitting power of the system is a direct function of the power of the received echo signal. If an echo signal is not received, because there is no reflecting obstacle in front of the vehicle, then the default emitting power must be selected to be high, in order to cover as large an area as possible in front of the vehicle and to be able to detect obstacles in this area. Therefore, an object appearing suddenly is struck by an unnecessarily intense scanning beam. A high emitting power must also be selected in the case of poorly reflecting obstacles.
In addition, German Published Patent Application No. 197 07 936 describes a method for determining a distance of an obstacle to a vehicle, using an optical distance sensor, where the emitting power of the distance sensor is controlled as a function of the traveling speed, in order to increase eye safety.
It is an object of the present invention to provide a device having a scanning laser sensor, and a method for operating such a device, which, on the average, consume less power over time, without losing considerable amounts of information.